Science Experiment: Surface Tension - Sand Castles

If you have ever tried to build a sand castle you know that there is a certain kind of wet sand that is perfect for it. If there is too much water in your bucket the mixture is too soupy. If you don't have enough water in your bucket the sand won't hold a shape and just crumbles. Why does the perfect mixture of sand and water work? Surface tension. Surface tension is the attraction that happens between water molecules. Water molecules are attracted to each other. The surface of water has an elastic quality because the molecules are hugging close together. This is why some insects can walk on water.

Water is made up of two kinds of atoms, hydrogen and oxygen. The name for the water molecule is H20. The water molecule has 2 hydrogen atoms and 1 oxygen atom. Water molecules are attracted to each other because hydrogen atoms and oxygen atoms are attracted to each other and hug close together really tight. This is called cohesion. The molecules hug so close together they don’t want to touch other molecules around them. That’s why a bubble or a drop of water is round and only rests a small part of itself on a surface when it lands.

When you add sand to water, the surface tension of the water forms little elastic bridges between the grains of sand. When the ratio of sand to water is just right these bridges are the perfect strength for building sand castles. In today's experiment you will be able to watch these bridge at work and figure out the best recipe for building sand castles.

What You Need:

• 12 Dixie Cups
• Sand
• Water
• 25 Pennies
• 4 Large Plates
• Large Bowl
• Measuring Cups (1/4, 1/3, 1/2, 1)

You are going to test what ratio of sand to water is the best one for building a strong sand castle. Label each plate - label the first one 1/4 cup, the second one 1/3 cup, the third one 1/2 cup and the last one 1 cup. For each trial you are going to use 1 cup of sand. The variable in this experiment is going to be the amount of water you add to the sand. For the first trial mix 1 cup sand and 1/4 cup water in the bowl. Fill three dixie cups with this mixture and turn them over to make small sand castles in the plate labelled 1/4 cup. Do the castles flatten or stay formed like the dixie cup? If any of them stay formed, stack pennies on top of the little castle one at a time until the little castle collapses. Write down how many pennies each little castle could hold. Repeat this test using 1 cup sand and 1/3 cup water, 1 cup sand and 1/2 cup water and 1 cup sand and 1 cup water. Keep track of your results on a chart like this:

One cup of sand to 1/3 cup water is what worked for us!

It turns out that water molecules attract to each other and they ALSO attract to sand. If you have a good balance of sand to water...nice and sticky...then you get a strong sand castle. If there is too much sand the mixture is too dry and the castle crumbles. If there is too much water the mixture is too wet and oozes all over the place.

Here is a fascinating video from space that demonstrates surface tension:

Websites, Activities & Printables:

• NASA: The Physics of Sandcastles
• Sand Castle Physics Revealed
• Sand Castle Central
• USGS Water Science School: Surface Tension
• Video: NASA STEMonstrations - Surface Tension
• Video Khan Academy: Surface Tension

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